do NOT attempt to clean with acids of any kind. Vinigar ain't gonna clean that either.
POR 15 stuff works great, problem is the rust converter they have is in a quart bottle, and you need a lot more than a quart, because of where the rust actually is in your tank. likely every surface below the fuel level should be ok, you need to pull the tank and remove the pump to see inside.
all you work is going to be from the bottom, thru that big hole....
I would however use the POR rust converter after using electrolysis to remove the major rust. the POR stuff turns rust into a phosphate coating, and prepares it for using the POR tank coating product, which you will now need anyway, as it has rusted once already.
http://www.por15.com/POR-15-Metal-Prep_p_15.htmlheres the complete kit, but it has the small bottle of converter..
http://www.por15.com/POR-15-Motorcycle-Fuel-Tank-Repair-Kit-_p_106.htmlthe actual coating should have enough in that kit, I did my KZ tank, 3+ gallon tank, and by rolling the tank around to fully coat it, when I drained the remainder out I had almost half a can extra.
it will also convert rust, but the way yours looks... it will need a lot, and will need to sit and slosh and sit again.
The metal prep product is reusable, so don't pour it out, it lasts a long time if it's not completely contaminated...
I'm going thru this right now with a friend and COGr on ADV who is restoring a tank right this second using a method I posted over a year ago...
this works....
http://www.zggtr.org/index.php?topic=15531.msg189401;topicseen#msg189401for a tank, you are gonna need to use big pieces of rebar, with rubber stoppers on either end creating a barbell looking probe, to keep it from shorting out on the innernsurface of the tank. this process takes time, but works amazingly...
read the process, and adapt it accordingly.... the positive lead goes to the scarificial rod, tyhe negative to the tank....
here's a pictorial and instructions ifound specific to bike tanks, I never saw it when I was doing my parts, but it shows some tips...
http://m.instructables.com/id/Electrolysis-rust-removal-motorbike-fuel-tank/best of luck...
I a do it yourselfer, I'd personally do the stuff I noted before popping big $$